BATF is the founding member of the BATF family of basic leucine zipper proteins and a major contributor to the function of AP-1 transcription factor complexes in immune cells. Batf null (Batf KO) mice are viable, yet display a number of phenotypes that position the protein within pathways critical for the regulation of inflammation, autoimmunity, host protection against pathogens and the prevention of lymphoproliferative disorders. As a result, there is enormous interest in using BATF, its regulators, and its effector genes in targeted strategies to control these health concerns. However, in order to assess which strategy to pursue, it is critical to identify targetable determinants that control BATF function with an emphasis on those determinants that discriminate between BATF function in different cellular contexts. This proposal is designed to address this gap in our knowledge. What is known is that BATF forms heterodimers with the JUN AP-1 proteins and that JUN:BATF dimers regulate genes by binding classic AP-1 DNA, or by recruiting IRF transcription factors to composite DNA elements called AICE. Both target gene activation and repression are associated with BATF expression in cells, but the precise manner by which these functions are distinguished has not been fully described. Additionally, there is evidence that BATF proteins can impact gene expression in the absence of DNA binding and that two phosphorylation events alter BATF target gene regulation by impacting its DNA binding and dimerization properties. The goal of this proposal is to test the hypothesis that phosphorylation of BATF selectively modulates BATF protein function in vivo. In Aim 1, BATF proteins engineered to mimic phosphorylation events and containing additional mutations that disrupt association with IRF4 and EGR2, will be assayed for DNA binding properties and for the ability to influence transcription using both AP-1 and AICE regulated genes. In Aim 2, the behaviors characterized in Aim 1 will be tested for their function in three, different, Batf-dependent transcription progras by examining BATF variants for their ability to rescue Batf KO T cell phenotypes. Completion of these aims will address our immediate goals of defining how naturally-occurring post-translational modifications impact the molecular properties of BATF and influence their competency to direct specific T cell functions. Results from this R21 will set the stage for future work to identify the kinase/phosphatase network(s) directing these BATF modifications in vivo and to profile how the cellular transcriptome responds following BATF modification. Reversible phosphorylation is among the most successful approaches for targeted drug design. A small molecule approach aimed at manipulating BATF post-translational modifications to inhibit, or augment, BATF function in vivo, will be a valuable tool for the management of immune system disorders.